Micro-lens configuration for small lens focusing in digital imaging devices

ABSTRACT

An improved image sensor wherein a first micro-lens array comprised of one or more micro-lenses is positioned over a cavity such that incoming light is focused on the photo sensors of the image sensor. The first micro-lens array may collimate and focus incoming light onto the photo sensors of the image sensor, or may collimate incoming light and direct it to a second micro-lens array which then focuses the light onto the photo sensors. A method of fabricating the improved image sensor is also provided wherein the cavity and first micro-lens array are formed by use of a sacrificial material.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.10/667,390, filed on Sep. 23, 2003, the subject matter of which isincorporated in its entirety by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to micro-lenses for imagerdevices and to methods for producing the same.

2. Brief Description of the Related Art

Image sensing devices (i.e., image sensors) are known that employ lightdetecting elements (e.g., photo sensors) for use in various applicationssuch as in semiconductor devices. Such image sensors may be formed usinga variety of fabrication techniques. Currently, two commonly fabricatedimage sensors are CMOS image sensors and charge coupled device (CCD)image sensors. Each generally includes an array of pixels containing thephoto sensors. The image sensors typically use photo sensors in the formof photogates, phototransistors or photodiodes.

When an image is focused on the imager array, light corresponding to theimage is directed to the pixels, including the photo sensors. It isknown in the art to use an imager array of pixels having a micro-lensarray that includes a convex micro-lens for each pixel. Each micro-lensmay be used to direct incoming light through a circuitry region of thecorresponding pixel to the photo sensor region, thereby increasing theamount of light reaching the photo sensor and thereby increasing thefill factor of the pixels. Each photo sensor is located at a suitabledepth in the image sensor such that light, when collimated and focusedby a corresponding micro-lens of a given focal length, is directed ontothe photo sensor. Examples of other uses of micro-lens arrays includeintensifying illuminating light from pixels of a non-luminescent displaydevice (such as a liquid crystal display device) to increase thebrightness of the display, forming an image to be printed in a liquidcrystal or light emitting diode printer, and providing focusing forcoupling a luminescent device or a receptive device to an optical fiber.

Typically, each pixel in the imager array produces a signalcorresponding to the intensity of light impinging on the photo sensorassociated with that pixel. The magnitude of the signal produced isapproximately proportional to the amount of light impinging on the photosensor. The signals may be used, for example, to display a correspondingimage on a monitor or to otherwise provide information about the opticalimage.

As semiconductor devices and the corresponding image sensors are madesmaller, the micro-lens arrays must be made smaller as well. As eachmicro-lens is made smaller, the focal length of the micro-lens isreduced. This reduced focal length causes light to be focused at areduced depth in the image sensor device. Typically, in prior artdevices, the depth of the photo sensors is reduced in the substrate tocorrespond to the reduced focal length of the micro-lenses. Decreasingthe photo sensor's depth in the substrate, however, requiresmanufacturing changes, and results in additional effort, expense andtime in processing.

Known fabrication methods for micro-lenses use patterning and reflowtechniques that are typically used to produce convex micro-lenses. Thesefabrication techniques are difficult to tailor to produce micro-lensesthat direct light to a particular focal point when the micro-lens sizeand geometry (and resultant focal length) change.

Additionally, in known image sensors, a large amount of light incidenton the image sensor device is not directed to the photo sensor due tothe geometry of the micro-lens array. In particular, light incident onthe space between individual micro-lenses (the lens-lens space) remainsuncaptured by the micro-lens, and never impacts the photo sensor. It isdifficult to produce image sensor micro-lenses with improved lightcapture using known patterning and micro-lens reflow processes.

Further, in known image sensors, a passivation layer such as a siliconnitride passivation layer separates the color filters from the photosensors. This passivation layer causes undesirable reflection in theoptical path to the photo sensors.

Accordingly, there is a desire and need for an image sensor havingmicro-lenses formed to focus light onto the photo sensors of the imagesensor such that the depth of the photo sensors does not have to bedecreased even when the image sensor device, including its photo sensorsand micro-lenses, decreases in size.

There is also a need for an improved method of fabricating an imagesensor micro-lens of desired geometry and focal length. Furthermore,there is a need for an image sensor including micro-lenses havingimproved light capture. Moreover, there is a need for an image sensorand method having reduced reflection in the current stack of the imagesensor.

SUMMARY OF THE INVENTION

The present invention provides micro-lenses formed to focus light ontothe photo sensors of the image sensor such that the depth of the photosensors does not have to be decreased even when the image sensor device,including its photo sensors and micro-lenses, decrease in size.

In one embodiment, an image sensor includes a first micro-lens array ofone or more first micro-lenses for collimating (i.e., capturing)incoming light and a second micro-lens array of one or more secondmicro-lenses for focusing the light onto a photo sensor. The secondmicro-lens array is separated from the first micro-lens array by acavity. The second micro-lens array and first micro-lens array arepositioned relative to one another such that incoming light iscollimated (i.e., captured) by a first micro-lens of the firstmicro-lens array, and then directed through the cavity to a secondmicro-lens of the second micro-lens array, which focuses the incominglight onto a corresponding photo sensor. In a preferred embodiment, thecavity that separates the second micro-lens array from the firstmicro-lens array consists of an air gap.

In another embodiment of the present invention, an image sensor includesa first micro-lens array of one or more first micro-lenses forcolliminating (i.e., capturing) and focusing incoming light onto a photosensor. The first micro-lens array is separated from the upper-mostlayers of the semiconductor substrate by a cavity.

Various embodiments of the image sensor may further include supports inthe form of posts, or in the form of other structures such as walls, tosupport the upper layers of the image sensor, including the image sensorlayers above the cavity, relative to the lower layers of the imagesensor. The supports may be located at any suitable location in theimage sensor such as at the edges of the image sensor or at locationsinterior of the edges of the image sensor.

In one embodiment of a fabrication method of the invention, a first ofat least two arrays of micro-lenses maybe formed by coating asemiconductor substrate with a layer of second micro-lens material and alower photo resist. Lower layer openings are developed in the photoresist. The second micro-lens material is then etched to form a secondmicro-lens array. A sacrificial material is applied to the secondmicro-lens array. Lens molds and one or more support molds are formed inthe sacrificial material. Supports are formed by filling the supportmolds with a support material that is at least substantiallynon-reflective and that is of sufficient strength to support the upperlayers of the image sensor. The first micro-lens array is formed byapplying a layer of first micro-lens material to the sacrificialmaterial to fill the lens molds formed in the sacrificial material. Thesacrificial material is removed from the image sensor by a suitablemethod (such as by heating or chemical removal) leaving a cavity betweenthe second micro-lens array and the first micro-lens array.

In a desired embodiment, the sacrificial material is degraded by heat orchemical treatment, and, as it degrades, the sacrificial materialdiffuses through the micro-lens material.

In an alternative embodiment, additional clean techniques are applied toremove residual particles through vacuum channels formed in the imagesensor. The sacrificial material may be removed, such as through heat orchemical treatment. Residual sacrificial material remaining after suchtreatment may be removed by vacuuming through the vacuum channels. Thevacuum channels may also be used to vacuum remove other residualparticles, such as residual chemicals that may remain after a chemicalremoval treatment.

A color filter array may be applied to the first-most micro-lensmaterial of the present invention. For example, a color filter may beapplied on top of the first micro-lens array. Additional coatings, forexample protective coatings, may also be applied on top of the colorfilter array. The color filter may also be applied at alternativelocations, for example below the second micro-lens array (i.e., thecolor filter array may be applied to the upper layers of the substratebefore applying the second micro-lens material).

In another embodiment of a fabrication method of the invention, an imagesensor is provided having a first micro-lens array for collimating andfocusing incoming light onto a photo sensor. The first micro-lens of thearray may be formed by coating a semiconductor substrate with a layer ofsacrificial material. Lens molds and support molds are formed in thesacrificial material. Supports are formed by filling the support moldswith a support material that is at least substantially non-reflectiveand sufficiently strong enough to support the upper layers of the imagesensor. A layer of first micro-lens material is applied to thesacrificial material to fill the lens molds and form the firstmicro-lens array. It is desired that the supports be made of the firstmicro-lens material and be formed at the same time as the firstmicro-lens array by applying the first micro-lens material. Thesacrificial material may be removed by heating or chemical treatment. Ifnecessary, one or more vacuum channels may be formed in the image sensorthrough which residual particles (e.g., sacrificial material or residualchemical solvents) may be removed.

The use of a sacrificial material in the present invention allowsfabrication of micro-lenses of a desired focal length such that lightmay be directed to corresponding photo sensors at a desired depth in asemiconductor substrate.

The geometry of the micro-lenses fabricated using the sacrificialmaterial lens molds can be formed to a variety of shapes. Preferably,the micro-lenses formed according to the invention have a concave shape.Concave micro-lenses may provide improved light capture, such as forexample where the edges of the micro-lenses of a given micro-lens arraytouch or overlap in the surface plane of the array.

Further, the image sensor may be constructed without a passivation layerbetween the color filter array and the photo sensors where the cavity ofthe image sensor is positioned between the color filter array the photosensors (such as where the color filter array is formed above thecavity).

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the invention will be moreclearly understood from the following detailed description which isprovided in connection with the accompanying drawings, in which:

FIG. 1 is an enlarged cross-sectional view of a conventional imagesensor including photo sensors, color filters and multiple micro-lensesformed thereon;

FIG. 2 is an enlarged cross-sectional view of the conventional imagesensor of FIG. 1, reduced in size including photo sensors, color filtersand micro-lenses of reduced size;

FIG. 3 is an enlarged cross-sectional view, in accordance with anexemplary embodiment of the invention, of an image sensor includingphoto sensors, color filters and multiple micro-lens arrays formedthereon;

FIG. 3 a is plan view of an exemplary embodiment of the image sensordevice of the invention having support walls;

FIG. 3 b is an enlarged cross-sectional view of an exemplary embodimentof the image sensor device of the invention having supports locatedinternal of the edges of the image sensor;

FIG. 4 is an enlarged cross-sectional view, in accordance with anexemplary embodiment of the invention, of an image sensor includingphoto sensors, color filters and a single micro-lens array formedthereon;

FIG. 5 a is an enlarged cross-sectional view of the embodiment of theimage sensor of FIG. 3 during fabrication, having a second micro-lensmaterial applied;

FIG. 5 b is an enlarged cross-sectional view of the embodiment of theimage sensor of FIG. 3 during fabrication, having second micro-lensesformed therein;

FIG. 5 c is an enlarged cross-sectional view of the embodiment of theimage sensor of FIG. 3 during fabrication, having a sacrificial materialapplied;

FIG. 5 d is an enlarged cross-sectional view of the embodiment of theimage sensor of FIG. 3 during fabrication, having a support openings andsacrificial resist openings formed therein;

FIG. 5 e is an enlarged cross-sectional view of the embodiment of theimage sensor of FIG. 3 during fabrication, having support molds formedtherein;

FIG. 5 f is an enlarged cross-sectional view of the embodiment of theimage sensor of FIG. 3 during fabrication, having lens molds formedtherein;

FIG. 5 g is an enlarged cross-sectional view of the embodiment of theimage sensor of FIG. 3 during fabrication, having a first micro-lensarray and supports formed therein;

FIG. 5 h is an enlarged cross-sectional view of the embodiment of theimage sensor of FIG. 3 during fabrication, having a first micro-lensarray and supports formed therein of different materials;

FIG. 5 i is an enlarged cross-sectional view of the first micro-lensarray and second micro-lens array of an embodiment of the image sensorof FIG. 3 during fabrication, having sacrificial material exposed at aside plane;

FIG. 6 a is an enlarged cross-sectional view of a stage of fabricationof vacuum channels for an image sensor, having a vacuum photo resistapplied thereto;

FIG. 6 b is an enlarged cross-sectional view of a stage of fabricationof vacuum channels for an image sensor, having vacuum openings formedtherein;

FIG. 6 c is an enlarged cross-sectional view of a stage of fabricationof vacuum channels for an image sensor, having vacuum channels formedtherein;

FIG. 6 d is an enlarged cross-sectional view of a stage of fabricationof vacuum channels for an image sensor, having plugged vacuum channels;

FIG. 7 a is an enlarged cross-sectional view of the embodiment of theimage sensor of FIG. 4 during fabrication, having a substrate provided;

FIG. 7 b is an enlarged cross-sectional view of the embodiment of theimage sensor of FIG. 4 during fabrication, having a sacrificial materialapplied;

FIG. 7 c is an enlarged cross-sectional view of the embodiment of theimage sensor of FIG. 4 during fabrication, having a sacrificial photoresist applied;

FIG. 7 d is an enlarged cross-sectional view of the embodiment of theimage sensor of FIG. 4 during fabrication, having a sacrificial resistopenings, and support openings formed therein;

FIG. 7 e is an enlarged cross-sectional view of the embodiment of theimage sensor of FIG. 4 during fabrication, having a support mold formedtherein;

FIG. 7 f is an enlarged cross-sectional view of the embodiment of theimage sensor of FIG. 4 during fabrication, having lens molds formedtherein;

FIG. 7 g is an enlarged cross-sectional view of the embodiment of theimage sensor of FIG. 4 during fabrication, having a first micro-lensarray and supports formed therein;

FIG. 8 a is an enlarged cross-sectional view of a stage of fabricationof an image sensor having a sacrificial resist applied thereto;

FIG. 8 b is an enlarged cross-sectional view of a stage of fabricationof an image sensor, having a lens molds and support molds formedtherein;

FIG. 9 a is an enlarged cross-sectional view of lens molds formed duringfabrication of micro-lenses;

FIG. 9 b is an enlarged cross-sectional view of the lens molds of FIG. 9a increased in widths;

FIG. 10 is an enlarged cross-sectional view of an exemplary embodimentof the invention, having convex lens;

FIG. 10 a is an enlarged cross-sectional view of the embodiment of theimage sensor of FIG. 10 during fabrication, having a sacrificial resistapplied thereto;

FIG. 10 b is an enlarged cross-sectional view of the embodiment of theimage sensor of FIG. 10 during fabrication, having convex lens moldsformed therein;

FIG. 10 c is an enlarged cross-sectional view of the embodiment of theimage sensor of FIG. 10 during fabrication, having a first micro-lensarray formed thereon; and

FIG. 11 is a schematic diagram of a processor system incorporating animage sensor comprising a plurality of micro-lenses constructed inaccordance with an exemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and which illustratespecific embodiments of the present invention. These embodiments aredescribed in sufficient detail to enable those of ordinary skill in theart to make and use the invention. It is also understood thatstructural, logical, or procedural changes may be made to the specificembodiments disclosed without departing from the spirit and scope of thepresent invention.

The terms “wafer” and “substrate” are to be understood asinterchangeable and as including silicon, silicon-on-insulator (SOI) orsilicon-on-sapphire (SOS), doped and undoped semiconductors, epitaxiallayers of silicon supported by a base semiconductor foundation, andother semiconductor structures. Furthermore, when reference is made to a“wafer” or “substrate” in the following description, previous processsteps may have been utilized to form regions, junctions or materiallayers in or on the base semiconductor structure or foundation. Inaddition, the semiconductor need not be silicon-based, but could bebased on silicon-germanium, germanium, gallium arsenide, or other knownsemiconductor materials.

The term “pixel” refers to a photo-element unit cell containing aphoto-conversion device or photo sensor and transistors for processingan electrical signal from electromagnetic radiation sensed by thephoto-conversion device. It should be understood that the invention maybe used with any pixel configuration such as e.g., three transistor(3T), four transistor (4T), five transistor (5T), etc., configuration.Although the invention is described herein with reference to thearchitecture of a defined number of pixels, such as one pixel or threepixels, it should be understood that this is representative of aplurality of pixels in an array of an imager device. In addition,although the invention is described below with reference to a CMOSimager, the invention has applicability to any solid state imagingdevice having pixels. The following detailed description is, therefore,not to be taken in a limiting sense, and the scope of the presentinvention is defined only by the appended claims.

Referring to FIG. 1, a prior art image sensor 1 is shown having photosensors 2 located in a semiconductor substrate 3. Micro-lenses 4 areformed on the semiconductor substrate 3. The image sensor 1 alsoincludes a color filter array 5. The focal length for the micro-lenses 4is shown as W. The photo sensors 2 are located at a depth in thesemiconductor substrate 3 such that incoming light Z is focused bymicro-lenses 4 onto the photo sensors 2.

FIG. 2 shows an image sensor 6 which is the image sensor 1 of FIG. 1fabricated to a reduced size. Photo sensors 7 (shown embedded insemiconductor substrate 8), micro-lenses 9, color filter array 10, andfocal length X illustrated in FIG. 2, are each reduced in size relativeto the photo sensors 2, micro-lens 4, color filter array 5, and focallength W shown in FIG. 1. The photo sensors 7 shown in FIG. 2, however,are at the same depth in the semiconductor substrate 8 as the photosensors 2 of FIG. 1. As shown in FIG. 2, the reduced focal length X,caused by the reduced size of the micro-lens 9, stops above the photosensors 7. Thus, incoming light Z is not focused onto the photo sensors7, which is undesirable.

FIG. 3 shows an embodiment of an image sensor 11 according to a firstexemplary embodiment of the invention. The illustrated image sensor 11includes a first micro-lens array 14 comprised of one or more firstmicro-lenses 15 and a second micro-lens array 12 comprised of one ormore second micro-lenses 13. The second micro-lens array 12 and firstmicro-lens array 14 are separated by a cavity 16. Preferably, the cavity16 is comprised of an air gap. In operation, incoming light Z iscaptured by the first micro-lenses 15 of first micro-lens array 14, anddirected through the cavity 16 to the second micro-lenses 13 of thesecond micro-lens array 12.

The illustrated imaging device 11 further includes pixel cells 37located in substrate 22. The illustrated pixel cells 37 comprise photosensors 19. In FIG. 3, photo sensors 19 are shown in the form of aphotodiode 38. It should be appreciated, however, that the photodiode 38may be any photosensitive region including a photogate or the like, andthe invention is not limited to the illustrated pixel cell 37. The pixelcells 37 are illustrated as a four-transistor (4T) pixel cell. It shouldbe noted that this illustration is not intended to limit the inventionto a particular pixel cell configuration. That is, the pixel cell maycontain, for example, three, four, five, or more transistors.

Each illustrated pixel cell 37 contains a photodiode 38 comprised of adoped region 38 a (typically a p-type region) formed within anotherdoped region 38 b (typically an n-type region). It is known in the artthat, in operation, a control signal (not shown) may be applied to thephotodiode 38 so that when incoming light Z in the form of photonsstrikes the photodiode 38, the photo-generated electrons accumulate in adoped source region under the photodiode 38. A transfer gate 40 islocated next to the photodiode 38, and has a floating diffusion region41 and a source/drain region 42. A field oxide layer 43 around the pixelcell 37 serves to isolate it from other pixel cells 37 in the array 36.

The transfer gate 40 is controlled by a transfer signal. Thesource/drain region 42 passes charges received from the photodiode 38 toan output transistor 44, a row select transistor 45 and then to readoutcircuitry 46. A reset transistor 47 operates to reset the floatingdiffusion region 41 to a predetermined initial voltage just prior tosignal readout. It should be noted that each pixel cell 37 contains anoutput transistor 44 and row select transistor 45 that are alsoelectrically connected to the readout circuitry 46, but are omitted fromFIG. 3 for the sake of clarity (the output transistor 44, row selecttransistor 45, and readout circuitry 46 are also omitted from subsequentdrawings for the sake of clarity of the drawings).

The image sensor 11 may include additional layers. For example,additional processing methods may be used to form insulating, shielding,and metallization layers to connect gate lines and other connections tothe pixel sensor cells. Also, an additional passiviation layer can beformed underneath the metallization layers. The passivation layer couldbe formed of, for example, silicon nitride, silicon dioxide,borosilicate glass (BSG), phosphosilicate glass (PSG), orboro-phospho-silicate glass (BPSG), which is CMP planarized and etchedto provide contact holes. These contact holes are then metallized toprovide contacts. Generally, a passivation layer is positioned betweenthe color filter array 17 and photo sensors 19, This passivation layeris not required, however, if the cavity 16 according to the presentinvention is positioned between the photo sensors 19 and the colorfilter array 17.

Conventional layers of conductors and insulators may also be used tointerconnect the structures and to connect the pixels to peripheralcircuitry. For the sake of clarity, the upper-most layers of thesubstrate 22 (including for example any insulating, shielding,metallization layers, and passiviation layer) will collectively bereferred to as an M1 layer of the semiconductor substrate 22.

In operation, incoming light Z is collimated by a first micro-lens 15 offirst micro-lens array 14 and directed through cavity 16 to acorresponding second micro-lens 13. The second micro-lens 13 focusesincoming light Z onto a corresponding photo sensor 19. Although FIG. 3and subsequent figures depict the incoming light Z as being generallyperpendicular to the semiconductor substrate 22, it may actually beincident at other angles and yet focused onto photo sensor 19. Prior topenetrating the first micro-lens array 14, the incoming light Z ispreferably filtered through a color filter array 17 positioned above thefirst micro-lens array 14, as shown in FIG. 3. The color filter array 17is comprised of three colored filters (e.g., 17 a, 17 b, 17 c). Typicalcolor filters 17 a, 17 b, 17 c include red, green, and blue filters(RGB), or cyan, magenta, and yellow (CMY) filters. It should be notedthat the color filters 17 a, 17 b, 17 c could also be a fluorescentmaterial film or other device for converting the wavelength of incidentlight. Each color filter 17 a, 17 b, 17 c, is respectively associatedwith a corresponding first micro-lens 15 of first micro-lens array 14.It is desirable to form the color filters 17 a, 17 b, 17 c on top offirst micro-lens array 14.

As shown in FIG. 3, image sensor 11 may further include a protectivelayer 18 formed on top of the color filter array 17 as the uppermostlayer of the image sensor 11. Preferably, the protective layer 18 iscomprised of an oxide layer, such as SiO₂, of thickness sufficient toprotect the image sensor from damage (for example, during packaging). Inaddition, layer 18 may be an antireflection coating (an “ARC”).

Although the color filter array 17 is shown in FIG. 3 as positionedabove the first micro-lens array 14, the color filter array 17 mayinstead be located at other areas in the image sensor 11, such as forexample between the M1 layer of the substrate 22 and the secondmicro-lens array 12. If the color filter array 17 is located at aposition below the first micro-lens array 14, then the first micro-lensarray 14 may be the uppermost layer of the image sensor 11.

The second micro-lenses 13 and first micro-lenses 15 may be comprised ofany suitable micro-lens material. Examples of known micro-lens materialsinclude silicon dioxide (SiO₂), silicon nitride, plasma enhancedchemical vapor disposition (PECVD) oxides (such as for example siliconoxides), interlayer dielectric materials, and BoroPhosphoSilicate Glass(BPSG), all of which are suitable for transmitting light. It is desiredthat the second micro-lenses 13 and first micro-lenses 15 are between0.5 and 1.5 microns at their thickest points and, in the surface plane,are about 2.0 to 5.2 microns in diameter.

Image sensor 11 further includes one or more supports 20. The supports20 are located at one or more designated areas capable of supporting theupper layers of the image sensor 11 (including the first micro-lensarray 14) relative to the second micro-lens array 12. The supports 20are preferably comprised of the same material as the first micro-lensarray 14, but alternatively can be comprised of a different materialthat is at least substantially non-reflective and that is sufficientlystrong enough to support the upper layers of the image sensor 11. Forexample where SiO₂ is the first micro-lens material, a suitable materialfor the supports 20 (other than the first micro-lens material) is, forexample, silicon nitride. Preferably, the supports 20 are in the form ofposts , as shown in FIG. 3, but may also be of other configurations suchas, for example in the form of support walls (best shown in FIG. 3 a).In addition, the single image sensor 11 may use supports 20 of differentforms, for example the image sensor 11 may use both posts and supportwalls . In FIG. 3, supports 20 are in the form of posts and are locatedat the edges 21 of the image sensor 11.

FIG. 3 b illustrates an alternative embodiment in which posts arelocated internally of edges 21. In FIG. 3 b, the supports 20 arepositioned within cavity 16 between the second micro-lens array 12 andthe first micro-lens array 14 at locations at least substantiallyoutside of the area within cavity 16 through which light is directedfrom the first micro-lenses 15 to the second micro-lenses 13. Forexample, referring to FIG. 3 b, first outer edges 15 a of firstmicro-lens 15 and second outer edges 13 a of second micro-lens 13 areusually not used for collimating and focusing light Z. The supports 20may, for example, be positioned between first outer edges 15 aand secondouter edges 13 a as shown in FIG. 3 b.

FIG. 4 illustrates another exemplary embodiment of the invention inwhich the image sensor of FIG. 3 is varied to include a singlemicro-lens array, in the form of a first micro-lens array 14. In thisillustrated embodiment, the first micro-lens array 14 both collimatesthe incoming light and focuses it onto the photo sensors 19. Referringto FIG. 4, a color filter array 17 is located on top of the upperlayers, i.e., the M1 layer, of the substrate 22. The first micro-lensarray 14 is positioned over the color filter array 17. A cavity 16separates the first micro-lens array 14 from the color filter array 17.One or more supports 20 are located at the edges 21 and support theupper layers of image sensor 11, including the first micro-lens array14, relative to the color filter array 17 and the M1 layer. The supports20 may alternatively be located at other positions at leastsubstantially outside the area within cavity 16 through which light Z isdirected to the lower layers of the image sensor 11 (e.g., to colorfilter array 17). The supports 20 may be formed of a material that is atleast substantially non-reflective and sufficiently strong to supportthe upper layers of image sensor 11. Preferably, both the supports 20and the micro-lens array 14 are comprised of SiO₂.

The manner of fabricating the image sensor illustrated in FIG. 3, is nowdescribed with reference to FIGS. 5 a through 5 i. Referring to FIG. 5a, a semiconductor substrate 22 having upper layers (the M1 layer) andphoto sensors 19 is provided. The M1 layer is coated with a secondmicro-lens material 23, such as for example with SiO₂. The secondmicro-lens material 23 is preferably applied using plasma enhancedchemical vapor disposition techniques known in the art for applyingoxides (preferably silicon oxides). The second micro-lens material 23may also be applied by other conventional micro-lens application methodssuch as by spin coating the second micro-lens material onto the M1layer, followed by soft-bake techniques. A suitable soft-bake techniqueknown in the art is to heat the micro-lens material at approximately 100degrees Celsius for approximately 1.5 minutes. A lower photo resist 24is applied on top of the second micro-lens material 23. The lower photoresist 24 may be applied by conventional methods such as spin coating.

It is known in the art that semiconductor substrates may include an M1layer having an uppermost layer comprised of a suitable micro-lensmaterial, such as for example an SiO₂ layer formed as an interlayerdielectric in the semiconductor substrate. In the illustratedembodiment, if the semiconductor substrate 22 were formed in suchmanner, the step of applying a second micro-lens material 23 is notrequired in order to fabricate the image sensor 11 embodied in FIG. 3.

Referring to FIG. 5 b, the lower photo resist 24 is exposed and areas ofthe lower photo resist 24 are developed away to form lower layeropenings 25. The lower photo resist 24 may be exposed by conventionalmethods such as by masking and then chemically developing away desiredareas of the photo resist, leaving lower layer openings 25.

As also shown in FIG. 5 b, the second micro-lens array 12 including oneor more second micro-lenses 13 is formed in the second micro-lensmaterial 23 (FIG. 5 a). Preferably, the second micro-lens array 12 isformed by etching second micro-lenses 13 using known etching methods,such as by applying a chemical etching solution followed by a rinse tostop the etching process. Etching the second micro-lens material 23forms the second micro-lens array 12 comprised of concave micro-lenses13. Preferably, the chemical etching solution is an isotropic etchsolution (i.e., a solution that etches the second micro-lens material 23at the same rate in all directions). An isotropic chemical etch forms aconcave lens of hemispherical shape.

Referring to FIG. 5 c, the second micro-lens array 12 is coated with asuitable sacrificial material 26. Examples of suitable sacrificialmaterials include materials that can be suitably etched and removed fromthe image sensor, such as for example photosensitive polycarbonates. Thesacrificial material 26 may be applied by conventional applicationmethods such as by spin coating the sacrificial material onto the secondmicro-lens array 12. A sacrificial photo resist 27 is applied on top ofthe sacrificial material 26. The sacrificial photo resist 27 may also beapplied by conventional methods such as spin coating.

Referring to FIG. 5 d, the sacrificial photo resist 27 is exposed andareas of the sacrificial photo resist 27 are developed away to formsacrificial resist openings 28. The sacrificial photo resist 27 may beexposed by conventional methods such as by masking and developing awaydesired areas of the photo resist 27, to leave the sacrificial resistopenings 28.

As also illustrated in FIG. 5 d, one or more support openings 51 isformed in the sacrificial photo resist 27. The sacrificial photo resist27 may be exposed by conventional methods such as by masking anddeveloping away desired areas of the photo resist, leaving supportopenings 51. Preferably, support openings 51 are formed in conjunctionwith the step of forming the sacrificial resist openings 28.

As shown in FIG. 5 e, the support molds 52 are formed in the sacrificialmaterial 26 and may be etched by conventional methods, such as byapplying a chemical etching solution followed by a rinse. Preferably, ananisotropic chemical etch is used to etch columnar support molds 52,that correspond to the width and length of the support openings 51 inthe surface plane, and that are etched through the sacrificial material26 to the second micro-lens array 12.

As illustrated in FIG. 5 f, lens molds 29 are also formed in thesacrificial material 26. In a preferred embodiment of the invention, thelens molds 29 may be etched by applying a chemical etching solutionfollowed by a rinse to stop the etching process. Preferably, thechemical etching solution is an isotropic etch solution. An isotropicchemical etch forms a concave lens mold 29 of hemispherical shape.

Referring to FIG. 5 g, supports 20 are formed by filling support molds52 (shown in FIG. 5 f) with a suitable support material. Suitablesupport materials are those that are a least substantiallynon-reflective and that are strong enough to support the upper layers ofthe image sensor 11 (FIG. 3). In addition, the first micro-lens array14, including one or more first micro-lenses 15, is formed by coatingthe sacrificial material 26 (including the lens mold 29 formed therein),thereby filling the lens molds 29, with a first micro-lens material 30,such as for example an SiO₂ layer.

In a desired embodiment, both supports 20 and first micro-lens array 14are comprised of the first micro-lens material 30. In accordancetherewith, both the supports 20 and the first micro-lens array 14 areformed by coating the sacrificial material 26 with the first micro-lensmaterial 30. The first micro-lens material 30 fills the support molds 52to form the supports 20, and also fills the lens molds 29 to form thefirst micro-lens array 14.

Alternatively, as shown in FIG. 5 h, supports 20 may be comprised of amaterial other than the first micro-lens material (used to form firstmicro-lens array 14) that is sufficiently non-reflective and strong. Forexample if SiO₂ is the first micro-lens material, a suitable materialfor forming the supports 20 may be, for example, silicon nitride. Insuch case, supports 20 are preferably formed by filling the supportmolds 52 with the support material before the lens molds 29 are filledwith the first micro-lens material 30.

The supports molds 52 may be located at any position such that thesupports 20 will be positioned to help support the first layers of theimage sensor 11 (including first micro-lens array 14) such as, forexample, at edges 21 as shown in FIG. 5 f. Preferably, as shown in FIG.5 g, the supports 20 are comprised of one or more posts positioned atthe edges 21 of the image sensor 11. As shown in FIGS. 5 h and 5 i,posts may be interspersed at the edges 21 along a side plane 54 of theimage sensor 11.

In an alternative embodiment of the invention, posts may be positionedinternally of edges 21 (as illustrated in FIG. 3 b) at areas withincavity 16 at least substantially outside of the areas wherein incominglight Z is directed from the first micro-lenses 15 to the secondmicro-lenses 13.

There may be circumstances (due to e.g., equipment limitations) in whichit is not feasible to selectively control the application of chemicaletching solutions to etch the support molds 52 and the lens molds 29 ina sacrificial material 26 using a sacrificial photo resist having bothsupport openings 51 and sacrificial resist openings 28 developedtherein. In such a case, it is desired to form the support openings 51and to etch the support molds 52 (e.g., using an anisotropic etchingsolution) prior to forming sacrificial resist openings 28 and etchinglens molds 29. After forming support molds 52, a new layer ofsacrificial photo resist 27 may be applied to the sacrificial material26 and sacrificial resist openings 28 developed in the sacrificial photoresist 27, and lens molds 29 etched there through (e.g., using anisotropic etch). The sacrificial photo resist 27 may be stripped, ifneeded, using known stripping techniques. It should be noted that thesacrificial resist openings 28 and lens molds 29 may alternatively beformed prior to forming the support openings 51 and etching supportmolds 52.

Additionally, if the support material is not the first micro-lensmaterial 30, there may also be circumstances (such as those due toequipment limitations) in which it is not feasible to selectivelycontrol the application of the support material or the first micro-lensmaterial 30 to a sacrificial material 26 having both the support molds52 and the lens molds 29 formed therein. In such a case, support molds52 are preferably formed prior to forming lens molds 29. Support molds52 may then be filled with the support material to form supports 20, asshown in FIG. 5 h. Excess support material may be removed from thesacrificial material using conventional techniques such as by applyingchemical solutions and planarization techniques. Lens molds 29 may thenbe formed and filled with the first micro-lens material 30 to form thefirst micro-lens array 14 (shown in FIG. 5 h). Alternatively, lens molds20 may be formed and filled (and excess first micro-lens material 30removed from the sacrificial material by conventional techniques such asby using chemical solutions or planarization) prior to forming andfilling support molds 52.

Referring to FIG. 5 g, sacrificial material 26 is removed to form cavity16 which provides the embodiment shown in FIG. 3. The sacrificialmaterial 26 may be removed by heat or chemical removal methods such asby heating the sacrificial material 26 or, in the case of a chemicalsacrificial material to which a resist has been applied, by treatingsacrificial material 26 with solvents or chemicals to remove the resistand sacrificial material 26. As would be known by those of ordinaryskill in the art, manufacturers of sacrificial materials may providerecommended removal methods (e.g., heat or chemical) and correspondingprocess conditions (e.g., temperature or time of removal treatment) fora given sacrificial material.

In a preferred embodiment of the invention, the sacrificial material 26is removed by heating the sacrificial material to its degradation point.Such heat removal technique may produce a very clean removal such thatno additional clean steps are required to remove residual particles.

There may be situations in which a single clean technique (such as heator chemical removal) does not provide a sufficiently clean image sensor.In such cases, additional clean steps may be desired or required toremove residual sacrificial material or, to remove other residualparticles (for example, particles that may remain after a chemicalremoval). In accordance with an alternative embodiment, additional cleansteps, may be provided by the formation and use of vacuum channels 31.

The formation of vacuum channels 31 is now described with reference toFIGS. 6 a through 6 c. Referring to FIG. 6 a, vacuum openings 35 may beformed by applying a vacuum photo resist layer 34 on top of the firstmicro-lens array 14. The vacuum photo resist layer 34 may be applied byconventional methods, such as for example by spin coating methods.

As shown in FIG. 6 b, the vacuum photo resist layer 34 is exposed, andone or more vacuum openings 35 are developed therein. The vacuum photoresist layer 34 may be exposed by conventional methods such as bymasking and developing away desired areas of the photo resist, leavingvacuum openings 35. The vacuum openings 35 should be formed above thenon-collimating portions of the first micro-lens array 14. Referring toFIG. 6 c, vacuum channels 31 are formed by etching through the firstmicro-lens array 14 to the sacrificial material 26. The etching processmay be by conventional means, such as by chemical etching. Preferably,the etching process forms vacuum channels 31 of substantially columnarshape. For example, an anisotropic chemical etch may be used to producesubstantially columnar shaped vacuum channels 31. After formation of thevacuum channels 31, it is desirable to remove the vacuum photo resist 34such as by conventional stripping.

The vacuum channels 31 may then be used to remove residual particlesthat remain after degrading the sacrificial material. The vacuumchannels 31 are formed after forming the first micro-lens array, butpreferably before cavity 16 is formed by removing sacrificial material26 (e.g., before degrading the sacrificial material such as by heatingor chemical treatment). After the sacrificial material 26 is removed toform cavity 16, it is desirable to plug the vacuum channels 31 byapplying a conventional filler material 48, such as a glue, to the outeropening of the vacuum channels 31 as shown in FIG. 6 d.

The vacuum channels 31 alternatively may be located at other areas inthe image sensor 11. If the vacuum channels 31 are formed by fabricatinga channel or opening through a micro-lens array of the image sensor 11,the vacuum channels 31 should be located in areas that are at leastsubstantially outside of the non-collimating and non-focusing areas ofthe micro-lens array.

As shown in FIG. 5 i, if the sacrificial material 26 is exposed at areasalong one or more side planes 54 of the image sensor 11, thensacrificial material 26 may be removed by vacuuming through one or moreexposed areas. Exposed areas of sacrificial material 26 may be formed ina side plane 54 of an image sensor 11, for example, when a series (i.e.,multiple) image sensors 11 are fabricated during processing. It iscommon when fabricating a series of image sensors 11, that a side plane54 of one image sensor 11 is connected to a side plane 54 of an adjacentimage sensor 11 until the final stages of fabrication, at which timeeach image sensor 11 is separated from the adjacent image sensor 11. Asa result of this separation, a side plane 54 of an image sensor 11 mayhave areas of exposed sacrificial material 26, as shown in FIG. 5 i,through which the sacrificial material 26 may be removed by vacuuming.

The method of fabricating the embodiment of the invention shown in FIG.4 is now described with reference to FIG. 7 a through FIG. 7 g. As shownin FIG. 7 a, a semiconductor substrate 22 is provided having an M1layer. As shown in FIG. 7 b, a sacrificial material 26 is applied on topof the M1 layer. It is preferred however, that the color filter array 17is first provided on top of the M1 layer and then the sacrificialmaterial 26 applied on top of color filter array 17. Examples ofsuitable sacrificial materials include materials that can be suitablyetched and removed from the image sensor, such as for examplephotosensitive polycarbonates. The sacrificial material 26 may beapplied by conventional application methods such as by spin coating thesacrificial material onto the color filter array 17.

As shown in FIG. 7 c, a sacrificial photo resist 27 is applied on top ofthe sacrificial material 26. The sacrificial photo resist 27 may also beapplied by conventional methods such as spin coating.

Referring to FIG. 7 d, the sacrificial photo resist 27 is exposed andareas are developed away to form sacrificial resist openings 28. Thesacrificial photo resist 27 may be exposed by conventional methods suchas by masking and developing away desired areas of the photo resist,leaving sacrificial resist openings 28.

As also illustrated in FIG. 7 d, the sacrificial photo resist 27 isexposed and areas developed away to form one or more support openings51. The sacrificial photo resist 27 may be exposed by conventionalmethods such as by masking and developing away desired areas of thephoto resist, leaving support openings 51. Preferably, the supportopenings 51 are formed in conjunction with the step of formingsacrificial resist openings 28.

Referring to FIG. 7 e, support molds 52 are formed in the sacrificialmaterial 26. The support molds 52 may be etched by conventional methods,such as by applying a chemical etching solution followed by a rinse.Preferably, an anisotropic chemical etch is used to etch columnarsupport molds, corresponding to the width and length (in the surfaceplane) of the support openings 51, through the sacrificial material tothe adjacent lower level. Preferably this adjacent lower level is thecolor filter array 17 as shown in FIG. 7 e, but different configurationsare also applicable. For example, the adjacent lower level may be the M1layer.

As illustrated in 7 f, lens molds 29 are formed in the sacrificialmaterial 26. In a preferred embodiment of the invention, lens molds 29may be etched by applying a chemical etching solution followed by arinse to stop the etching process. Preferably, the chemical etchingsolution is an isotropic etch solution (i.e, a solution that etches thesacrificial material 26 at the same rate in all directions). Anisotropic chemical etch forms a concave lens molds 29 of hemisphericalshape.

Referring to FIG. 7 g, supports 20 are formed by filing support molds 52(shown in FIG. 7 f) with a suitable support material. Suitable supportmaterials are those that are a least substantially non-reflective andthat are strong enough to support the upper layers of the image sensor11. Supports molds 52 (FIG. 7 f) may be located at any positions suchthat supports 20 formed thereby are capable of helping to support thefirst layers of the image sensor 11, including the first micro-lensarray 14. As also shown in FIG. 7 g, lens molds 29 (illustrated in FIG.7 f) are filled with the first micro-lens material 30 to form the firstmicro-lens array 14.

Preferably, both supports 20 and first micro-lens array 14 are comprisedof the first micro-lens material 30. In such case, supports 20 and firstmicro-lens array 14 may be formed by coating the sacrificial material 26with the first micro-lens material 30. The first micro-lens material 30fills the support molds 52 to form the supports 20 and fills the lensmolds 29 and to form the first micro-lens array 14. Preferably, thefirst micro-lens material 30 is SiO₂.

Alternatively, supports 20 may be formed separately of the firstmicro-lens array 14. For example, supports 20 may be formed before thelens molds 29 are filled with the first micro-lens material 30, in whichcase, supports 20 may be comprised of a material other than the firstmicro-lens material 30. Suitable materials are those that are at leastsubstantially non-reflective and that have sufficient strength tosupport the upper layers of the image device 11 including the firstmicro-lens array 14. Where, for example, the first micro-lens material30 is SiO₂, a different suitable support material such as siliconnitride may be used.

Preferably, as shown in FIG. 7 g, the supports 20 are comprised of oneor more posts positioned at the edges 21 of the image sensor 11. Ifsupports 20 are located internally of the edges 21, support molds 52(shown in FIG. 7 f) should be positioned at locations that are at leastsubstantially outside of the areas through which light is directed fromthe first micro-lens array 14 to the lower levels of the image sensor 11(e.g., to the color filter array 17).

In fabricating the embodiment shown in FIG. 4, the sacrificial material26 (shown in FIG. 7 g) is removed by heat or chemical treatment to formthe cavity 16 illustrated in FIG. 4. If additional clean techniques arenecessary or desired, vacuum channels 31 may be formed by etchingthrough the first micro-lens array to the sacrificial material aspreviously described with reference to FIGS. 6 a-6 d, or to FIG. 5 i.

The fabrication of the lens molds 29 and support molds 52 of theembodiments shown in FIG. 3 and FIG. 4 has been described by photoresist and etching steps. Another method suitable for fabricating thelens molds 29 and support molds 52 of the FIG. 3 and FIG. 4 embodimentsis now described with reference to FIGS. 8 a-8 b. According to thismethod, a sacrificial photo resist and subsequent masking and etching ofa sacrificial photo resist are not required. As shown in FIG. 8 a, thesacrificial material 26 is applied to the image sensor (e.g., to thesecond micro-lens layer when fabricating the embodiment of FIG. 3, or tothe color filter array when fabricating the embodiment illustrated inFIG. 4). As shown in FIG. 8 b, the sacrificial material may then beetched using conventional controlled laser etching techniques, such asfor example stereo lithography techniques, to form lens molds 29 andsupport molds 52.

The present invention provides a simple apparatus and method forfocusing light on a photo sensor without decreasing the depth of thephoto sensor in the semiconductor wafer. Examples of suitableapplications include digital imaging applications wherein light istransmitted to a light device of a pixel cell, and in digital imagedisplay applications wherein light is transferred from a pixel cell of adisplay device. The invention is particularly suitable for use inintegrated circuit applications using semiconductor devices such as forexample CMOS or CCD devices.

The method of forming micro-lens arrays through use of a sacrificialmaterial 26 provides micro-lenses with a desired geometry, and providesa cavity of a desired depth, such that the focal point of themicro-lenses is at a desired point in the substrate (i.e., is at thephoto sensors). For example, when using an isotropic etch to form firstmicro-lenses 15 of a concave shape, the geometry of such micro-lensesmay be adjusted by varying the size of the sacrificial resist openings28 through which the lens molds are etched in the sacrificial material26, and by controlling the exposure time of the chemical etchingsolution used to etch said lens molds 29. When using an isotropic etch,the amount of sacrificial material etched in the surface plane of thesacrificial material may be varied by varying the size of thesacrificial resist opening 28. Applying the chemical etching solution toa narrower sacrificial resist opening 28 a will form a lens mold 29 a(shown in FIG. 9 a) that is narrower in the surface plane of thesacrificial material 26, but that is at the same depth perpendicular tothe surface plane, relative to a lens mold 29 b (shown in FIG. 9 b) thatis formed when the resist solution is applied to a wider sacrificialresist opening 28 b.

The amount of sacrificial material 26 applied during fabrication can becontrolled to provide a desired cavity depth, and a desired distancebetween the first micro-lens array 14 and the lower levels of the imagesensor 11 (e.g., the color filter array 17 in the FIG. 4 embodiment orthe second micro-lens array 12 in the FIG. 3 embodiment). For example,in a multi-array configuration like that shown in FIG. 3, using moresacrificial material 26 can create a first micro-lens array 14 that islocated further away from the second micro-lens array 12 than if lesssacrificial material 26 were used. Thus, an image sensor 11 having adesired focal point to which the first micro-lens array 14 and thesecond micro-lens array 12 direct light within the substrate 22 themicro-lenses can be provided.

Additionally, the depth of the lens molds 29 (and the depth of thecavity 16 that remains after the lens molds 29 are filled andsacrificial material 26 is removed), may be controlled by controllingthe exposure time of the etching process.

The lens configurations of the present invention are preferably concaveconfigurations, but can include other lens configurations, such as forexample the convex configuration shown in FIG. 10. FIG. 10 is a singlearray configuration that comprises a first micro-lens array 14 havingconvex first micro-lenses 15.

Convex first micro-lenses 15 may be formed by controlled laser etching(for example by stereo lithography) of the sacrificial material 26 afterit is applied to the image sensor. The embodiment illustrated in FIG. 10is formed by applying the sacrificial material 26 to the upper layer(e.g., the M1 layer) of a substrate 22 (shown in FIG. 10 a), thenforming convex lens molds 29 (such as by controlled laser etching) inthe sacrificial material (shown in FIG. 10 b), and forming a convexfirst micro-lens array 14 by filling the lens molds 29 with firstmicro-lens material 30 (shown in FIG. 10 c), then removing thesacrificial material 26 to produce the image sensor of FIG. 10.

If multiple micro-lens arrays are desired for the image sensor 11, asecond micro-lens array 12 having convex second micro-lenses 13 may beformed prior to applying the sacrificial material 26. Convex micro-lensarrays may be formed by known micro-lens fabrication methods (notillustrated herein). A known method for forming second micro-lenses ofconvex shape is by applying a lower photo resist 24 to the secondmicro-lens material 23, then masking and exposing away part of the lowerphoto resist 24 to develop first openings therein, then heating thesecond micro-lens material 23 and lower photo resist 24 according toconventional methods such that the surface tension caused by thepresence of the undeveloped lower photo resist 24 causes the underlyingsecond micro-lens material 23 to form a second micro-lens array 12 ofconvex micro-lenses 13. A sacrificial material 26 may be applied to thesecond micro-lens array 12. Then first micro-lens array 14 and cavity 16a may then be formed in the manner described with reference to FIGS. 10,and 10 a-10 c (except that sacrificial material 26 is applied to thesecond micro-lens array rather than to the upper layer (e.g., M1 layer)of substrate 22).

FIG. 11 illustrates an exemplary processing system 600 that may utilizethe image sensor of the invention. For example, the image sensor 11 ofFIG. 3, incorporating first micro-lens array 14 and second micro-lensarray 12 constructed in accordance with the embodiment of the inventionillustrated in FIGS. 5 a-5 i may be part of the processing system 600.Other image sensors may be used in accordance with the presentinvention, for example, the image sensor 11 of FIG. 4, incorporatingfirst micro-lens array 14 (constructed in accordance with the embodimentof the invention illustrated in FIGS. 7 a-7 g) may be used. Any one ofthe electronic components shown in FIG. 11, including CPU 601 and imagesensor 11, may be fabricated as an integrated circuit for use inprocessing images.

The processing system 600 includes one or more processors 601 coupled toa local bus 604. A memory controller 602 and a primary bus bridge 603are also coupled to the local bus 604. The processing system 600 mayinclude multiple memory controllers 602 and/or multiple primary busbridges 603. The memory controller 602 and the primary bus bridge 603may be integrated as a single device 606.

The memory controller 602 is also coupled to one or more memory buses607. Each memory bus accepts memory components 608 which include atleast one memory device 110. The memory components 608 may be a memorycard or a memory module. Examples of memory modules include singleinline memory modules (SIMMs) and dual inline memory modules (DIMMs).The memory components 608 may include one or more additional devices609. For example, in a SIMM or DIMM, the additional device 609 might bea configuration memory, such as a serial presence detect (SPD) memory.The memory controller 602 may also be coupled to a cache memory 605. Thecache memory 605 may be the only cache memory in the processing system.Alternatively, other devices, for example, processors 601 may alsoinclude cache memories, which may form a cache hierarchy with cachememory 605. If the processing system 600 includes peripherals orcontrollers which are bus masters or which support direct memory access(DMA), the memory controller 602 may implement a cache coherencyprotocol. If the memory controller 602 is coupled to a plurality ofmemory buses 607, each memory bus 607 may be operated in parallel, ordifferent address ranges may be mapped to different memory buses 607.

The primary bus bridge 603 is coupled to at least one peripheral bus610. Various devices, such as peripherals or additional bus bridges maybe coupled to the peripheral bus 610. These devices may include astorage controller 611, a miscellaneous I/O device 614, a secondary busbridge 615, a multimedia processor 618, and a legacy device interface620. The primary bus bridge 603 may also be coupled to one or morespecial purpose high speed ports 622. In a personal computer, forexample, the special purpose port might be the Accelerated Graphics Port(AGP), used to couple a high performance video card to the processingsystem 600.

The storage controller 611 couples one or more storage devices 613, viaa storage bus 612, to the peripheral bus 610. For example, the storagecontroller 611 may be a SCSI controller and storage devices 613 may beSCSI discs. The I/O device 614 may be any sort of peripheral. Forexample, the I/O device 614 may be a local area network interface, suchas an Ethernet card. The secondary bus bridge may be used to interfaceadditional devices via another bus to the processing system. Forexample, the secondary bus bridge 616 may be a universal serial port(USB) controller used to couple USB devices 617 via to the processingsystem 600. The multimedia processor 618 may be a sound card, a videocapture card, or any other type of media interface, which may also becoupled to one additional device such as speakers 619. The legacy deviceinterface 620 is used to couple legacy devices 621, for example, olderstyled keyboards and mice, to the processing system 600.

The processing system 600 illustrated in FIG. 11 is only an exemplaryprocessing system with which the invention may be used. While FIG. 11illustrates a processing architecture especially suitable for a generalpurpose computer, such as a personal computer or a workstation, itshould be recognized that well known modifications can be made toconfigure the processing system 600 to become more suitable for use in avariety of applications. For example, many electronic devices whichrequire processing may be implemented using a simpler architecture whichrelies on a CPU 601 coupled to memory components 608 and/or memorydevices 110. These electronic devices may include, but are not limitedto audio/video processors and recorders, gaming consoles, digitaltelevision sets, wired or wireless telephones, navigation devices(including system based on the global positioning system (GPS) and/orinertial navigation), and digital cameras and/or recorders. The imagingdevices of the present invention, when coupled to a pixel processor, forexample, may be implemented in digital cameras and video processors andrecorders. Modifications may include, for example, elimination ofunnecessary components, addition of specialized devices or circuits,and/or integration of a plurality of devices.

It should again be noted that although the invention has been describedwith specific references to CMOS pixel cells having a strained siliconlayer, the invention has broader applicability and may be used in anyimage sensor device. For example, the present invention may be used inconjunction with CCD imagers. Similarly, the processes described aboveare but only a few methods of many that may be used. The abovedescription and drawings illustrate preferred embodiments which achievethe objects, features, and advantages of the present invention. Althoughcertain advantages and preferred embodiments have been described above,those skilled in the art will recognize that substitutions, additions,deletions, modifications and/or other changes may be made withoutdeparting from the spirit or scope of the invention. Accordingly, theinvention is not limited by the foregoing description but is onlylimited by the scope of the appended claims.

1. An image sensor comprising: a plurality of pixels, each pixelcomprising: a first micro-lens formed over a substrate for capturingincoming light; and a second micro-lens formed over the substrate forreceiving the incoming light from the first micro-lens and focusing theincoming light onto a corresponding photo sensor formed within thesubstrate, wherein the first micro-lens is separated from the secondmicro-lens by a cavity, wherein a surface of one of the first and secondmicro-lenses abutting the cavity is convex and a surface of the other ofthe first and second micro-lenses abutting the cavity is concave.
 2. Theimage sensor of claim 1 wherein said cavity is filled with air.
 3. Theimage sensor of claim 1 wherein said first micro-lens is comprised of afirst micro-lens material selected from the group consisting of silicondioxide, silicon nitride, plasma enhanced chemical vapor deposition(PECVD) oxides, interlayer dielectric materials, and BoroPhosphoSilicateGlass (BPSG).
 4. The image sensor of claim 1 wherein said secondmicro-lens is comprised of a second micro-lens material selected fromthe group consisting of silicon dioxide, silicon nitride, plasmaenhanced chemical vapor deposition (PECVD) oxides, interlayer dielectricmaterials, and BoroPhosphoSilicate Glass (BPSG).
 5. The image sensor ofclaim 1 wherein the surface of the first micro-lens abutting the cavityis concave.
 6. The image sensor of claim 5 wherein the surface of thesecond micro-lens abutting the cavity is convex.
 7. The image sensor ofclaim 1 wherein the surface of the first micro-lens abutting the cavityis convex.
 8. The image sensor of claim 7 wherein the surface of thesecond micro-lens abutting the cavity is concave.
 9. The image sensor ofclaim 1 further comprising a colored filter formed on top of said firstmicro-lens.
 10. The image sensor of claim 9 further comprising aprotective layer positioned on top of the colored filter.
 11. The imagesensor of claim 3, further comprising one or more posts for supportingthe first micro-lens relative to the second micro-lens.
 12. The imagesensor of claim 11 wherein said one or more posts are positioned at oneor more edges of the image sensor.
 13. The image sensor of claim 11wherein said one or more posts are positioned internal to edges of theimage sensor.
 14. The image sensor of claim 13 wherein said one or moreposts are comprised of said first micro-lens material.
 15. The imagesensor of claim 3 wherein said one or more supports are one or moresupport walls positioned at one or more edges of the image sensor. 16.The image sensor of claim 15 wherein said one or more support walls arecomprised of said first micro-lens material.
 17. The image sensor ofclaim 1, further comprising one or more supports for supporting thefirst micro-lens relative to the second micro-lens.
 18. An imager systemcomprising: a processor; and an image sensor electrically coupled tosaid processor, said image sensor comprising a plurality of pixels, eachpixel comprising: a first micro-lens formed over a substrate forcapturing incoming light; a second micro-lens positioned along anoptical path for receiving the incoming light from the first micro-lensand focusing the incoming light onto a corresponding photo sensor,wherein the first micro-lens is separated from the second micro-lens bya cavity, wherein a surface of one of the first and second micro-lensesabutting the cavity is convex and a surface of the other of the firstand second micro-lenses abutting the cavity is concave; a colored filterpositioned along said optical path; and one or more supports forsupporting the first micro-lens relative to the second micro-lens. 19.The imager system of claim 18 wherein said cavity is filled with air.20. The imager system of claim 18 wherein said one or more supportscomprise one or more posts.
 21. The imager system of claim 20 whereinsaid one or more posts are positioned at one or more edges of the imagesensor.
 22. The imager system of claim 20 wherein said one or more postsare positioned internal to edges of the image sensor.
 23. The imagersystem of claim 22 wherein said one or more posts are comprised of thefirst micro-lens material.
 24. The imager system of claim 18 whereinsaid one or more supports are one or more support walls positioned atone or more edges of the image sensor.
 25. The imager system of claim 24wherein said one or more support walls are comprised of said firstmicro-lens material.